1. Field of the Invention
The present invention relates to a power tool configured to control the rotation speed of a motor in accordance with an amount of operation of a trigger switch.
2. Description of the Related Art
There are power tools which rotate tip tools such as a drill, driver, or a like by using a motor as a driving source. Of these types of power tools, there is a power tool which controls the rotation speed of a motor in accordance with an amount (degree) of operation of a trigger switch. In general, such power tool is configured to control the rotation speed of a motor by varying a voltage applied to the motor in accordance with an amount (degree) of operation (stroke) of the trigger switch.
Generally, the power tool of this type increases (or decreases) a voltage applied to the motor in accordance with an increase (or a decrease) in the amount of operation (stroke) of the trigger switch to exert control so that the rotation speed of the motor is raised (or decreased). Such the control prevents a rapid rise in the rotation speed of the motor at a time of start of operations and rotates the motor at a low speed to make it possible to easily position a tip tool in an object to be worked or to enhance ease of working.
A power tool to perform such control as above is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2000-024960. The power tool disclosed in the publication determines, in accordance with an ON/OFF state of a main contact of a trigger switch, whether the trigger switch has been operated or not. The power tool also determines the rotation speed of a motor based on a signal from a speed contact of the trigger switch. The speed contact changes output voltage thereof in accordance with the amount of operation (stroke) of the trigger switch.
Further, a power tool is proposed which uses a brushless motor in order to achieve long life of the power tool. Unexamined Japanese Patent Application KOKAI Publication No. 2007-196363, for example, discloses a power tool using a brushless motor.
When the start or stop of the motor is controlled in response to only an ON/OFF state of a main contact, there is a possibility that, regardless of whether an operator has moved his/her hands off the trigger switch, the main contact is turned OFF due to some reasons such as vibration, noise, or the like. When the main contact is turned OFF, the motor also stops regardless the operator has not moved his/her hands off the trigger switch.